In aviation, it is common to use boxes having selection and/or adjustment buttons, in particular in the cabin for selecting communications channels and/or for adjusting the corresponding volume level. There are thus on/off selection buttons, adjustment selection buttons, potentiometer-type adjustment buttons, transient contact buttons, or adjustment and transient contact buttons. To an ever-increasing extent, even when the buttons are designed for performing potentiometer-type adjustments, they are of the pushbutton type, thereby making it easy to distinguish the selected and the non-selected positions of each button.
In general, the selection and/or adjustment buttons used are constituted by a translucent body having a front face which is illuminated when light is being conveyed in the thickness of the front panel.
In the particular case of boxes for use on board aircraft, and in particular the cockpit, as much as possible is done to make it easy to identify buttons so that the user can select the appropriate button quickly and without effort.
The greater the contrast coefficient, the easier such identification becomes, where the coefficient is defined by the ratio (L.sub.B -L.sub.F).times.1/L.sub.F where L.sub.B designates the luminance of a point on the button and L.sub.F designates the luminance of a point on the front panel. The luminance at a point is generally measured by means of a spectrophotometer associated with a computer, the average being taken of measurements performed on a plurality of points on an object (the lower the luminance, the larger the number of points required).
Given the practical difficulty of including illumination within a button itself (because of limited space and high cost, and because of problems of heating), it is common practice to illuminate a button from within the thickness of the front panel, with the front face of the front panel and the visible portion of the side surface being painted (generally black, gray, or dark blue) so that the light inside the front panel cannot escape, thereby making it possible to illuminate only the front faces of buttons thus causing them to stand out more clearly from the front face of the front panel. In the increasingly common circumstance of pushbuttons, proposals have been made to attenuate the illumination of the front face of a button when it is pushed-in by using a narrow passage for diffusing light where the button passes through the front panel in association with a white-painted ring on the body of the button, said ring being located away from the narrow passage when the button is in its non-pushed-in position (button non-selected) and blocking said passage when the button is in its pushed-in position (button selected).
Using such a system, it has been possible to achieve nighttime contrast values of greater than 5, which is acceptable, however daytime contrast values in full sunlight remain poor (on average less than 1.5). Measurement is generally performed using a source providing a light intensity or illumination of 90,000 lux to 100,000 lux, at a specified incidence and distance.
Attempts have been made to improve the "full sunlight" daytime situation by providing a white collar on the body of the button that is visible from the side when the button is in its non-pushed-in position and that disappears when the button is pushed in. However, the contrast obtained is insignificant (i.e. not measurable) when the button is seen from in front under conditions of full sunlight contrast, such that results are still not satisfactory in such a situation.